This invention relates to sulfur detecting apparatus. When sulfur in some form such as sulfur dioxide is introduced into a hydrogen-air flame, it forms molecular sulfur, S.sub.2, which is excited and emits characteristic light in a series of bands in the blue and ultra violet region of the spectrum. In particular there are bands with wavelengths centered at about 364 nm., 374 nm., and 384 nm. At intermediate wavelengths near 370 nm. and 380 nm. there is little or no emission by the sulfur. The sulfur generated light is emitted diffusely from an extended volume around the flame rather than from a concentrated source. There is in addition to the sulfur emission a general flame background light emission which is independent of the presence of sulfur.
Optical band pass filters have been placed in the path of the light passing from the flame to a light detector to restrict the wavelengths to those of the sulfur generated light, minimize the background light passing to the detector, and so improve detection. Interference filters have been used since they are readily available with a narrow band pass. Interference filters have been unsatisfactory for this purpose, however, since the extended sulfur emitting volume around the flame provides light that is not practical to collimate while an interference type filter requires collimated light for proper operation. The alternative of using absorption type filters which are not sensitive to light direction is also unsatisfactory because known absorption filters have not had a narrow enough pass band to be useful.